Artificial inorganic peroxidase is of great interest due to its intrinsic advantages over natural counterpart. Negatively charged gold nanoparticles (AuNPs) were discovered to function like a peroxidase in the present study. Two AuNPs in different size were prepared and characterized by TEM, and assayed for peroxidase activity. Its catalytic activity was found to follow Michaelis-Menten kinetics. The negative surface charge notably improves the affinity toward a substrate TMB, proved by the determined kinetic parameters. The particles expressed optimal catalytic activity under mildly acidic environment and resistance to elevated temperature and increased concentration of sodium azide. The origin of the activity was investigated tentatively. Hydrogen peroxide-treated AuNPs exhibited an enhanced activity. EDTA temporarily blocked the activity partially, while thiol groups permanently blocked the activity completely. Tests imply that it is the surface Au+ that provides the activity. The successful oxidation of dopamine, as an instance, under the action of AuNPs as a peroxidase was conducted. These studies would lead to a wide range of potential applications.